Method for liquefaction of natural gas

Abstract

A method of altering the heating value of a liquefied natural gas by adding higher heating value components is disclosed. A portion of the liquefied natural gas is used to cool the higher heating value component stream prior to combining the higher heating value components with the liquefied natural gas to obtain a combined stream having a heating value greater than the liquefied natural gas.

1. A method of altering the heating value of a liquefied natural gas stream comprising:
 providing a first liquefied hydrocarbon composed of at least 90 wt % methane, having a temperature of about −150° C. or less and having a first heating value, in a first storage vessel; 
 providing a second liquefied hydrocarbon composed primarily of one or more of ethane, propane, and butane, or mixtures thereof; 
 providing a first stream of the first liquefied hydrocarbon from the first storage vessel; 
 diverting a first portion of the first stream and compressing it to form a compressed first stream; 
 passing the compressed first stream through the cold side of a first heat exchanger to obtain a heated first stream; 
 decompressing the heated first stream in a first liquid expander to obtain a decompressed first stream; 
 returning the decompressed first stream back to the first storage vessel; 
 providing a second stream of the second liquefied hydrocarbon; 
 compressing the second stream to form a compressed second stream having a temperature from about −40° C. to about −160° C.; 
 passing the compressed second stream having a temperature from about −40° C. to about −160° C. through the warm side of the first heat exchanger to cool the compressed second stream to obtain a cooled second stream; 
 combining a first portion of the cooled second stream with a second portion of the first stream to obtain an third stream having a second heating value greater than the first heating value of the first liquefied hydrocarbon. 
 
     
     
       2. The method of  claim 1 , wherein the first liquid expander provides static expansion to the heated first stream to obtain the decompressed first stream. 
     
     
       3. The method of  claim 1 , further comprising:
 diverting a second portion of the cooled second stream and decompressing it in a second liquid expander to form a second decompressed second stream; and 
 injecting the second decompressed second stream into the second stream. 
 
     
     
       4. The method of  claim 3 , wherein the second liquid expander provides static expansion to a portion of the cooled second stream to obtain the second decompressed second stream. 
     
     
       5. The method of  claim 1 , wherein the flow of the cooled second stream into the first stream is regulated to control the heating value of the third stream. 
     
     
       6. The method of  claim 1 , wherein the temperature of the cooled second stream is no more than 50° C. warmer than the temperature of the first stream. 
     
     
       7. The method of  claim 1 , wherein the high heating value of the third stream is 1050 Btu/SCF or greater. 
     
     
       8. The method of  claim 1 , wherein the pressure of the heated first stream is sufficient to keep the heated first stream in a liquid state. 
     
     
       9. The method of  claim 1 , wherein the temperature of the heated first stream is no warmer than −100° C. 
     
     
       10. The method of  claim 1 , wherein the temperature of the cooled second stream is cold enough to prevent cavitation as the cooled second stream is combined with the first stream. 
     
     
       11. The method of  claim 1 , wherein temperature approach on the heat exchanger between the heated first stream and the compressed second stream is less than 20° C. 
     
     
       12. The method of  claim 1 , wherein temperature approach on the first heat exchanger between the compressed first stream and the cooled second stream is less than 20° C. 
     
     
       13. The method of  claim 1 , further comprising:
 diverting a second portion of the cooled second stream and decompressing it in a second liquid expander to form a second decompressed second stream, wherein flow of the second decompressed second stream is regulated to maintain a temperature approach on the first heat exchanger between the heated first stream and the compressed second stream of less than 20° C. 
 
     
     
       14. The method of  claim 1 , further comprising:
 diverting a second portion of the cooled second stream and decompressing it in a second grid expander to form a second decompressed second stream, wherein flow of the ninth stream is regulated to maintain a temperature approach on the first heat exchanger between the compressed first stream and the cooled second stream at no more than 20° C. 
 
     
     
       15. The method of  claim 1 , wherein the temperature of the second stream is 0° C. or less. 
     
     
       16. The method of  claim 1 , wherein the temperature of the second stream is −30° C. or less. 
     
     
       17. A method of modifying the heating value of a liquefied natural gas stream comprising:
 providing LNG composed of at least 90 wt % methane, and having a first heating value in a first storage vessel; 
 providing LPG composed primarily of one or more of ethane, propane, and butane, or mixtures thereof; 
 passing a first stream of the LNG through a first heat exchanger to provide cooling energy and a warmed first stream of the LNG; 
 passing a stream of the LPG having a temperature from about −40° C. to about −160° C. through the first heat exchanger to obtain a cooled stream of the LPG; 
 providing a second stream of the LNG; 
 blending a first portion of the cooled stream of the LPG with the second stream of the LNG to obtain a third stream containing LNG and containing increased content of LPG and thus having a heating value greater than the first heating value of the LNG. 
 
     
     
       18. The method of  claim 17 , further comprising returning the warmed first stream of the LNG exiting the first heat exchanger to the first storage vessel. 
     
     
       19. The method of  claim 17 , wherein the temperature of the cooled stream of the LPG exiting the first heat exchanger is no more than 50° C. warmer than the temperature of the first stream of the LNG entering the first heat exchanger. 
     
     
       20. The method of  claim 17 , further comprising blending a second portion of the cooled stream of the LPG exiting the first heat exchanger into the stream of the LPG prior to entering the first heat exchanger. 
     
     
       21. The method of  claim 1 , wherein the compressed second stream, has a temperature from about −80° C. to about −120° C. 
     
     
       22. The method of  claim 17 , wherein the stream of the LPG has a temperature from about −80° C. to about −120° C.